Xerogels and aerogels are highly porous materials with a particularly low envelope density and high surface area. They typically also display exceptionally low thermal conductivity and acoustic propagation properties. As such, they are useful in a wide range of applications including as purification/separation media, non-reflective panels, gas storage media, catalyst support, porous substrates e.g. sponges and electrochemical device electrodes (for supercapacitors, fuel cells and lithium ion batteries).
The most common examples are silica aerogels usually made by sol-gel processes and carbon hydrogels obtained from pyrolysis of resorcinol-formaldehyde resin.
Carbon nanotubes are a new form of carbon with an intrinsically high aspect ratio and nanoscale diameter. Individually, they have high strength, high modulus, useful electrical conductivity, and large surface area. Attempts to exploit these properties in macroscopic form depend on the development of appropriate processing techniques.
In recent years, a number of attempts have been made to prepare carbon nanotube-based aerogels. One such example includes the creation of carbon nanotube aerogels from aqueous-gel precursors by critical-point-drying and lyophilisation (freeze-drying) (Carbon Nanotube Aerogels, Byrning M. B., Milkie D. E., Islam M. F., Hough L. A., Kikkawa J. M., Yodh A. G., Adv. Mater., 2007, 19, 661). This method involves the use of polyvinyl alcohol (PVA) to reinforce the carbon nanotube aerogels. Although such reinforced nanotubes display improved strength and stability, the presence of PVA invariably results in several disadvantages including increased parasitic mass and reduced electrical conductivity.
There have also been reports of carbon aerogels, using nanotubes as additional filler (Properties of Single-Walled Carbon Nanotube-Based Aerogels as a Function of Nanotube Loading, Worsley M. A., Pauzauskie P. J., Kucheyev S. O., Zaug J. M., Hamza A. V., Satcher Jr. J. H., Baumann T. F., Acta Materialia, 2009, 57, 5131). Although this method provides carbon aerogel composite foams with improved electrical properties, these foams suffer from large volumetric shrinkage during the drying and carbonisation steps, unless over 20 wt % of single-walled carbon nanotubes is present in the foams.
Furthermore, synthesis of cross-linked multi-walled carbon nanotube films has been previously reported (Relation of the Number of Cross-Links and Mechanical Properties of Multi-Walled Carbon Nanotube Films Formed by a Dehydration Condensation Reaction, Ogino S., Sato Y., Yamamoto G., Sasamori K., Kimura H., Hashida T., Motomiya K., Jeyadevan B., Tohji K., J. Phys. Chem., 2006, 110, 23159). However this method requires filtration of the nanotubes to form a dense film before cross-linking. Since the resultant cross-linked film is not a carbon aerogel, the film is less porous and has a much greater density than is desirable.
Physical gels of carbon nanotubes are known in solvents, caused by entanglement or weak non-covalent association; however, these gel networks are not robust, have low strength, and collapse in the absence of solvent (Gelation in Carbon Nanotube/Polymer Composites, Liu C. et al., Polymer, 2003, 44, 7529-7532).
Therefore, the present invention seeks to provide a method of obtaining cross-linked carbon nanotube networks, which are selected from aerogels and xerogels, which overcome the above-mentioned problems. The present invention also seeks to provide cross-linked carbon nanotube networks which are selected from aerogels and xerogels which allow more control over the density, shape, conductivity and internal surface of the nanotubes, so that they display desirable electrical and mechanical properties.